The combination of green thorax and blue abdomen distinguishes males (and some females) from most other dragonflies. The all-green, non-metallic patterning of females is characteristic. Among remaining species, the blue-eyed hawker (Aeshna affinis), green hawker (Aeshna viridis) and southern hawker (Aeshna cyanea) have a predominantly black, banded abdomen with blue and/or green spots, lack a dorsal stripe and have distinct black markings on the thorax. Lesser emperor (Anax parthenope) has a brown thorax and a yellow ring at the base of the second abdominal segment that persists into adulthood. Occasional individuals of vagrant emperor (A. ephippiger) may have blue abdomens, but never have blue eyes and the thorax is brown. Common green darner (Anax junius) may be encountered as an occasional vagrant from the Americas in Western Europe; this species lacks blue eyes, never has blue markings on the thorax, and has a distinct front marking resembling a bulls-eye, with a circular, black central mark surrounded by a blue ring.

The male has a strikingly sky blue abdomen which when in flight has a slightly downward curve, giving a bent appearance, a useful feature for separating the restless Emperor from the other hawker type dragonflies whose abdomens generally appear straight. The blue green eyes & green thorax make this a very handsome insect.

The female is generally green in appearance with dark markings along the centre of the abdomen. They oviposit alone into floating vegetation or submerged plant stems, a feature which helps to separate this species from the Lesser Emperor (Anax parthenope) which generally oviposits in tandem.

The eyes dorsally broadly contiguous, or dorsally narrowly apposed to dorsally broadly contiguous (in the female); brown and green, or green. Legs brownish black and yellow (the undersides of the fore-femora yellow). Thoracic antehumeral stripes absent. The wings spread more or less horizontally in repose; dissimilar in shape and venation; sessile; unpatterned and clear. The inner wing venation blackish. Discoidal cell divided longitudinally into a conspicuous triangle and supra-triangle. Antenodal veins in the forewings about 16-20 (fewer in the hindwings); incorporating two conspicuously stronger primaries, and those in the costal and subcostal spaces unaligned. Pterostigma narrow-linear; black, or dark brown. 

Abdomen linear from a conspicuously swollen base (briefly constricted behind the swelling in the male); 49-51 mm long (female), or 53-61 mm long (male); predominantly blue (male), or green (female); predominantly longitudinally lined (with a continuous, median black line which is crossed by a short band on each segment of the male, and on the anterior segments in the female); without mid-dorsal spots. The male abdomen without auricles on segment 2; with a single inferior anal appendage. 

The larvaes stout, the body expanded in the middle; when mature, 45-56 mm long. The eyes large; approaching one another closely at a point on the top of the head; markedly flattened dorsally, their posteror margins forming a transverse straight line in dorsal view (this and the more rounded outline of the head readily distinguishing this species from the Aeschnas). The head in dorsal view not markedly narrowing from immediately behind the eyes. The postocular lobes curving smoothly to the back of the head from immediately behind the eyes. The antennae 7 segmented. The mask narrowed gradually to the hinge; with a flat prementum; with a short slit-like median cleft. Legs shorter than the abdomen; fore- and middle tarsi 3-segmented. The abdomen terminating in five short spine-like appendages; gizzard with 4-8 folds. 

This powerful dragonfly is one of the largest species in Europe. The male is highly territorial, and difficult to approach. The species lives by larger ponds, gravel pits, and slow rivers. They frequently fly high up into the sky in search of prey, which includes butterflies, Four-spotted Chasers and tadpoles; small prey is eaten on the wing. The females lay the eggs into plants such as pondweed, and always lay alone.

In contrast to other European emperors (genus Anax), females lay eggs alone, ovipositing directly into floating vegetation, including deadwood, away from shore. This exposed situation makes animals highly visible compared with related species.

The eggs are laid in surface water-plants, and develop in the region of highest temperature and light intensity. They hatch in about 3 weeks. Ovi-position lasts for about 8 weeks each year, usually extending from early June to early August. The protracted period of oviposition and the direct development of the eggs combine to endow the hatching larval population with a wide temporal variation.

Duration of the larval stage is usually about 2 years, but exceptionally it may be completed in 1 year. Larval growth is restricted to about 6 months each year, from May to October. A few precocious larvae hatching from eggs laid early achieve the penultimate or final instars by the first autumn, and emerge as adults the following spring. The remainder, usually composing more than 90% of an emergence-group, attains an average length of about 24 mm (range 7-42 mm) before growth ceases in autumn. Growth is resumed in May, and the final larval instar is entered in August, approximately 12 months after hatching. The next 9 months are spent in the final instar without visible growth, and emergence takes place the following spring between mid-May and mid-July, about 23 months after hatching. 

Yearly emergence is restricted to a period of 40-50 days, usually extending from late May to early July, and about 90% of an annual population of adults emerge during the first 10 days of the emergence period. Over the whole emergence period males tend to emerge earlier than females. Emerging as an adult on reeds or rushes around the margins of their chosen water body, the emergence took place in the middle night, and newly emerged adults fly shortly before sunrise. For the maiden flight they fly away from water and may fly several hundred metres before alighting. This orientation is perhaps an adaptation directed towards protecting teneral adults from the aggression of sexually mature males. Adults fly again during the first day, and remain away from water for the next 2 weeks. The cast skin or exuvia can be found long after this spectacular transformation takes place and is an exact replica of the larvae which can be identified to species level. Collecting these exuviae can give an excellent indication of the true population size at a site and is conclusive proof of successful breeding.

Following emergence, maturation takes between a week and 12 days in males, 13-16 days in females. During this maturation period they fly often, feed, shun water and show no sexual response to other dragonflies. After a minimum of 9 days of adult life, the first males begin to exhibit sexual behaviour over water; females return to water to oviposit from 2 to 3 days later. Copulation occurs promptly upon arrival of a female, and is followed almost immediately by oviposition. Both sexes probably return to the water frequently after the first mating. Males are capable of copulating more than once. The expectation of life at emergence of those adults destined to survive the maturation period is probably about 4 weeks, and the maximum life-span approximately 8 1/2 weeks. Predators of mature adults are few. 

Flight season highly dependent on location, with North African populations flying from March until December. In northern Europe, animals are most abundant from June to August.